1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine and a printer. More specifically, the present invention relates to an image forming method that is applied to the apparatus and relates to an apparatus for performing image formation by converting light to scanning light using a reflective polyhedron.
2. Description of the Related Art
Conventionally, there have been known two methods as an image forming method for obtaining a color image on a sheet material by superimposing toner images of a plurality of colors. One is a method of forming electrostatic latent images on an image bearing member, sequentially developing them by toner to transfer them to a sheet material each time they are developed and superimposing toner images of a plurality of colors on the sheet material (hereinafter referred to as multiple transfer method). The other is a method of superimposing toner images of a plurality of colors on an image bearing member or an intermediate transfer member and transferring the superimposed toner images of a plurality of colors collectively to a sheet material (hereinafter referred to intermediate transfer method).
Among them, the former method is put to practical use with structures described in DE2607727, Japanese Patent Application Laid-open No. Sho 50-50935 and the like.
On the other hand, the latter method is a technology having advantages such as a simple structure and currently drawing attention, although it has problems of color mixture at the time of development and image deterioration at the time of re-transfer.
Moreover, in recent years, a so-called tandem type multiple transfer method, which is capable of forming a full color image using one path by providing process units such as a photosensitive member and a developing device independently for each color, begins to draw attention in terms of high-speed productivity.
In any of the methods, toner images T of a plurality of colors formed and superimposed on a sheet material 103 are finally heated and fixed by a fixing device that uses a roller or a belt as shown in FIG. 8. As a result, toner images of each color are mixed and a full color image can be obtained. FIG. 8 is a schematic view of a fixing device that is used in the conventional image forming apparatus.
An example of a fixing device of this type by a heat roller method will be hereinafter described with reference to FIG. 8. In FIG. 8, a fixing roller 101 is for melting to fix the toner T on the sheet material 103 by heating and application of pressure. A pressure roller 102, which is disposed opposing the fixing roller 101, is for applying pressure to the fixing roller 101.
Usually, the fixing roller 101 often has a structure to be heated by a heater 104 such as a halogen lamp from the inside of the roller.
Therefore, it is a general practice to monitor a surface temperature of the fixing roller 101 by a temperature sensing element 114 such as a thermistor to on/off control the heater 104, thereby performing temperature control.
In addition, as the toner T, toner of four colors of yellow, cyan, magenta and black is generally used.
In this case, silicone oil 106 or the like is applied to the surface of the fixing roller 101 by a blade 105 via a drawing roller 116 and an applying roller 115 in order to prevent offset. Alternatively, toner originally containing wax or oil component is used to prevent offset.
In addition, the surface of the fixing roller 101 may be cleaned by a cleaning web 107 or the like, if necessary.
Incidentally, in the case in which a transparent film for an over-head projector (hereinafter referred to as OHP (over-head projector) sheet), a sheet for realizing high gloss (hereinafter referred to as high gloss paper or high gloss sheet) or the like is used, a method of giving it a heat quantity larger than that required for fixing an image on a normal transfer sheet to perform fixing and realizing desired transparency and glossiness is often used.
As a method for this purpose, for example, it is possible to increase a heat value of the heater 104 incorporated in the fixing roller 101. However, with this method, a sufficient heat quantity is not obtained in many cases and it takes long to make a temperature of the fixing roller 101 follow. Thus, this method cannot always be effective.
As another method, there is a method of reducing a fixing speed by the fixing roller 101 to give a sufficient heat quantity to a sheet. With this method, a desired heat quantity is easily obtained and, at the same time, it is possible to execute fixing immediately without providing a waiting time because it is unnecessary to change a temperature setting of the fixing roller 101 itself.
However, if reduction in a fixing speed is attempted as described above, a sheet exists on a transfer portion and a fixing portion simultaneously unless a distance between the transfer unit and the fixing unit is sufficiently large as compared with a maximum sheet length of a sheet to be used (in case of an intermediate transfer member, unless a distance between a primary transfer unit and a secondary transfer unit is sufficiently large as compared with the maximum sheet length) in case of the above-mentioned multiple transfer method.
Consequently, a fixing speed cannot be reduced after toner images of final colors are formed on a photosensitive member until all toner images are transferred to the sheet.
That is, it has become necessary to additionally rotate a transfer drum or an intermediate transfer member. In this case, a printing time is substantially extended
Moreover, in recent years, a so-called quadruple multiple transfer method using photosensitive members for four colors draws attention as a high-speed full color printing method. In this method, decelerated fixing cannot be performed with a machine in which a distance between portions for transferring and fixing final color is shorter than a maximum sheet length unless the machine is put in a decelerated state in advance from the time when an image is formed on a photosensitive member. (It is needless to mention that, in the above-mentioned multiple transfer or intermediate transfer member method, it is also possible to perform the same method as described below instead of rotating the transfer drum or the intermediate transfer member additionally.)
That is, in an image forming apparatus with a short distance between a transfer unit and a fixing unit, it becomes necessary to perform image formation in a decelerated state in advance in order to avoid shock caused by reducing a fixing speed during image formation.
Nevertheless, the applicants and the like found, as a result of examining a laser printer of a method of deflecting a laser beam irradiated from a light source by a reflective polyhedron (e.g., polygon mirror) to scan a member to be scanned such as a photosensitive member by the deflected laser beam, that a laser beam quantity became excessive and deficiencies such as tendency of image block-up and defective middle tones were caused if image formation is performed in a decelerated state (e.g., at a half speed of a standard speed) in the laser printer.
Thus, it became necessary to reduce a laser beam quantity in the decelerated state. However, it was difficult to steadily realize a significant change of a light amount such as reducing it to half or one third as in this example using a laser beam emitting circuit.
In addition, it took long to change a speed of a polygon mirror and a process speed could not be changed promptly.
There is known a method of making predetermined surface unused among all surfaces of a polygon mirror instead of reduction of a laser beam quantity.
This method is a so-called thinning-out scanning for thinning out the number of surfaces to be used of a polygon mirror to use them and substantially reducing a laser beam quantity, which is recognized as a publicly-known technology. Further, thinning out one surface of reflective surfaces of a polygon mirror means that a reflective surface skipped one surface from a reflective surface used for optical scanning is used for the next optical scanning.
However, when the applicants or the like performed this thinning-out scanning, a new problem concerning an image occurred.
The problem is that dispersion of scanning lengths in the main scanning direction that each surface of a polygon mirror has with respect to an image having periodicity such as a dither image, so-called jitter, causes interference and fringe-like streak unevenness occurs on, for example, uniform images of middle tones.
This moirxc3xa9 phenomenon will be described with reference to FIGS. 4 to 7. FIG. 4 is a schematic view showing a relation between a structure of a polygon mirror to be used in the conventional image forming apparatus and scanning lengths. FIG. 5 is a conceptual view showing a dither matrix to be used in the conventional image forming apparatus. FIGS. 6 and 7 are schematic views showing image patterns to be formed in the conventional image forming apparatus.
As an example, the case in which a polygon mirror 121 has eight surfaces as shown in FIG. 4A will be described.
If a rotating shaft 123 of the mirror deviates with respect to a central position 122, influence in the main scanning direction appears as a infinitesimal deviation (undulation) of a scanning position from left to right of FIG. 4B as shown by scanning lines 11 to 18 corresponding to mirror surfaces S1 to S8 of FIG. 4B.
Here, starting positions for starting writing in the left end are aligned by synchronizing writing signals by known detecting means. However, in accordance with the rightward scanning, deviation of positions occurs for each surface of the polygon mirror, which becomes maximum in the right end.
That is, as the scanning moves from the left end to the right end, dots gradually deviate and the eight ends of the scanning undulate to left and right at a period of every eight scanning lines.
On the other hand, as an example of an image pattern, various dither matrix as shown in FIG. 5 are assumed and pixels are grown in an order indicated by numerical values (i.e., grown in a swirling shape outward from the center).
In this case, in printers of recent years, a so-called multiple value dither for further dividing the inside of one dot in the figure into a plurality of steps to grow pixels may be used.
In any case, when such a repeating pattern is used, growth centers as shown in FIG. 6 (represented by white circles or black circles) appear on a bit map.
Here, a few dither matrixes D based on the growth centers (represented by white circles or black circles) are shown as an example in FIG. 6.
When undulation of a beam position appearing at a period of every eight scanning lines due to the above-mentioned deviation of polygon surfaces is caused in this example, sparse parts and dense parts appear at a period of every eight scanning lines with approaching the right end on the image. Then, for example, the part of the black growth center corresponding to the scanning line 11 is enhanced in FIG. 6 (looks slightly darker than the other growth centers).
The parts of the black circles shown in FIG. 6 look black because the part with less vertical deviation has a closer distance between upper and lower dots than the other parts due to undulation in image formation of the polygon mirror shown in FIG. 4.
In other words, it can be said that the parts of the white circles of FIG. 6 are parts in which a distance between upper and lower dots was expanded (in a slant direction) at the time of image formation due to the undulation of the polygon mirror shown in FIG. 4 and the dots became sparse, as a result, the entire pixel relatively looks light.
It seems that the pixels look dark when each dot is concentrated and look light when each dot is dispersed because an amount of toner or a shape of a toner image is affected by a change of a depth of potential at the time of latent image formation by the electrophotographic system or an edge effect at the time of development.
In any case, shades of a pixel are caused by undulation in the main scanning direction by a polygon mirror of FIG. 4 and the shades occur at a period of this undulation.
However, as in this example, if the period of undulation (here, period of eight lines) is large as compared with a size of a dither matrix, since a distance between dots to be enhanced expands and neighboring enhanced dots do not look connected, a moirxc3xa9 pattern is rarely discerned on an image.
Nevertheless, when thinning-out scanning of a polygon mirror following reduction in an image forming speed as described above is performed, for example, if every other surface of a polygon mirror is used as the image forming speed is reduced to half, the above-mentioned undulation appears at a period of four scanning lines corresponding to half of the eight surfaces.
FIG. 7 shows how interference between a dither growth center and undulation occurs in this case. In FIG. 7, white circles and black circles are also centers of the dither matrixes shown in FIG. 5. That is, in FIG. 7, the white circles and the black circles are also centers of the dither matrixes D shown in FIG. 6.
As is evident from FIG. 7, as the period of undulation is reduced to four scanning lines, a distance between dots to be enhanced is narrowed and strong correlation as shown by L1 to L3 (i.e., a slant fringe pattern due to moirxc3xa9) is caused.
A degree of this moirxc3xa9 fringe pattern changes according to a magnitude of undulation of scanning by a polygon mirror, a scanning density for one dot, a shape of dither and the like. As an example, it was found that, when the scanning density was set to 600 dots/inch (in this case, a pixel by the above-mentioned dither was equivalent to approximately 120 lines), an interference fringe in the right end of the scanning by the polygon mirror was a slant line with a pitch of approximately 0.6 mm and, then, even if a manufacturing accuracy of a polygon mirror and a rotating shaft was increased to suppress a difference between a maximum value and a minimum value of an amount of deviation of dots in the main scanning direction causing undulation to be equal to or less than 1 dot, a moirxc3xa9 fringe at the time of thinning-out scanning due to deceleration was very conspicuous.
On the other hand, it was found that it was difficult in terms of manufacturing accuracy to reduce undulation caused by a polygon mirror to a degree at which a moirxc3xa9 fringe was not seen (e.g., ⅕ dot or less in the right end).
Further, this moirxc3xa9 fringe pattern tends to be conspicuous when a size of a dither (the number of dots in one side) and the number of surfaces of a polygon mirror are close. Since the number of lines of a dither that is often used in general is 100 to 200 (i.e., the number of dots in one side is 6 to 3 dots) and, on the other hand, a polygon mirror with four to twelve surfaces is often used, if polygon mirror surface is thinned out and used as described above, the moirxc3xa9 fringe pattern changes to be conspicuous.
The present invention has been devised in view of the above and other drawbacks, and it is an object of the present invention to provide an image forming apparatus and an image forming method that are capable of preventing decrease in a quality of an image to be formed even if reflective surfaces of a reflective polyhedron are thinned out in reducing an image forming speed.
Another object of the present invention is to provide an image forming apparatus comprising: a movable image bearing member; a light source; a rotatable reflective polyhedron for reflecting light from said light source and optically scanning said image bearing member; a first mode for forming an image on said image bearing member at a first speed; and a second mode for forming an image on said image bearing member at a speed of 1/(n+1) of the first speed, and when it is assumed that the number of surfaces of said reflective polyhedron is m, m/(n+1) is not an integer at the time of said second mode (provided that m is an integer of three or more and n is a natural number).
Still another object of the present invention is to provide an image forming method comprising the steps of: switching from a first mode for forming an image on an image bearing member at a first speed to a second mode for forming an image on an image bearing member at a speed smaller than the first speed; and using reflective surfaces of a reflective polyhedron, in which the number of rotatable surfaces is m, for reflecting light from a light source and optically scanning an image bearing member by skipping n surfaces such that m/(n+1) does not make an integer (provided that m is an integer of three or more and n is a natural number).
Further objects of the present invention will become apparent from the following explanation.